This ability is paid for during the declare attackers step of the combat phase. [Aahz 1995/01/27]

Paying Brainwash is not an instant or any other kind of ability, it is a cost increase on the declaration of the attacker. This makes it much like the way Gloom makes a white spell more expensive to cast. [WotC Rules Team 1998/03/01]

If there are multiple combat phases during the turn, the attack cost must be paid each time if you want to attack with the creature. [D'Angelo 1999/08/01]